This invention relates to a device for measuring surface roughness.
Known available devices for measuring surface roughness are divided into two groups. The first group comprises those able to make the measurement without direct contact between the device and examined surface, and the second group comprises those which use a sensing element, by means of direct contact, senses the surface unevenness.
Of the devices pertaining to the first group, particular use is made of the microscope, which can be provided with one or two lenses, or be of the interference type. In particular, the microscope with a single lens provides only one surface magnification, whereas that comprising two lenses enables the surface of the examined piece to be observed simultaneously with the surface of a sample piece. The interference microscope is provided with a lamp which emits monochromatic light for illuminating both the examined surface and a smooth comparison mirror simultaneously. Interference fringes form between the examined surface and the mirror, and the degree of roughness can be defined numerically by examining these fringes.
Devices pertaining to the second group generally comprise a feeler needle having a very small point radius, for example of the order of 1-2.mu., i.e. of dimensions of the same order as the unevenness of the examined surface. While the examined piece traverses slowly relative to the needle, the latter moves perpendicularly to the surface so that the surface becomes periodically probed. The movement of the feeler needle is measured for example electrically and possibly amplified, and the signal thus obtained is used to operate a writing point which reproduces on paper the variation in the profile of a predetermined section of the said surface.
While known devices of the aforesaid type are widely used for defining the degree of surface roughness, they have certain drawbacks. In particular, microscopes with one or two lenses only allow a qualitative judgement of the surface roughness, whereas an objective numerical evaluation is often required regarding the degree of roughness. In the case of interference microscopes and devices comprising a feeler needle, even though they provide numerical information regarding the degree of roughness, they require the use of highly specialized personnel. In this respect, in the case of the former it is necessary to evaluate the shape of the interference fringes, whereas in the case of the latter it is necessary to process the measured data relative to the profile.
Finally, none of the measuring devices of the said first or second group is suitable for use in rapidly evaluating surface defects in any zone of a workpiece being machined, because they are difficult to handle and are structurally too delicate.